The old £1 coins must have been ridiculously easy to fake. I once worked in an office where the vending machine gave change, and about 1 in every 10 £1 coins it gave out was obviously counterfeit. I made a little collection of them because I found them interesting.

The City of New York used to auction off all the slugs it collected from parking meters. Granted - no guarantee that they worked. But it always seemed short-sighted to me that all that illegal change you collected? ...yeah, let's get that right back into circulation.